I believe it is because at one point Drupal was a pretty solid foundation for a lot of project types and was greatly flexible.
Right now WordPress is way more popular though.
Right now WordPress is way more popular though.
Also, Drupal is often coined a CMS. If you google "drupal is not a cms" you'll find a number of people referring to it as CMF - content management framework. It really allows you to build a custom CMS for a specific purpose rather than customizing a CMS for a specific purpose. It may sound like semantics, but if/when you have a few Drupal sites under your belt you really get what they mean.
[edited for clarity]